This invention relates to a multiple inverter formed of a plurality of unit inverters in which an accidental or faulty overcurrent flow in the inverter circuit is prevented.
FIG. 1 shows a typical configuration of a prior art multiple inverter. An AC input voltage is applied via a breaker 10 to a rectifier (AC to DC converter) 12. Ripples of an output voltage of rectifier 12 are filtered out via an LC filter 13. The filtered DC output voltage from the LC filter 13 is applied to DC lines 15P and 15N. Two unit inverters 14 and 16 are connected in parallel to DC lines 15P and 15N. Each of the unit inverters 14 and 16 converts the inputted DC voltage into an AC output voltage. The AC output of unit inverter 14 is connected in series with the AC output of unit inverter 16, and the serially connected AC outputs of inverters 14 and 16 are applied via a high-speed circuit breaker 18 to a load 20 which may contain a counter-electromotive force source or any other electric power source. Where three or more unit inverters are used in a special case, the output of these unit inverters may be connected in series and/or parallel. An apparatus of such serially and/or parallel connected (stacked) unit inverters is called a multiple inverter. Multiple inverters are widely used where a large power-handling capacity is required or a low-distortion AC output is required.
FIG. 2 shows details of the configuration of conventional unit inverters. A positive DC line 15P is coupled via a series circuit of GTO (gate-turn-off) thyristors 141 and 142 to a negative DC line 15N. Line 15P is also coupled to line 15N via a series circuit of GTO thyristors 143 and 144. Diodes D141 to D144 are respectively anti-parallel-connected to thyristors 141 to 144. The juncture of thyristors 141 and 142 is coupled via a primary winding of an output transformer T14 to the juncture of thyristors 143 and 144. The elements 141-144, D141-D144 and T14 constitute a first unit inverter 14. A second unit inverter 16 coupled parallel to the first unit inverter 14 has the same configuration as the unit inverter 14.
When thyristors 141 and 144 are on and thyristors 142 and 143 are off, a positive DC potential +E of lines 15P and 15N is applied to a terminal U1 of the primary of transformer T14. Whereas, when thyristors 142 and 143 are on and thyristors 141 and 144 are off, a negative DC potential -E of lines 15P and 15N is applied to the terminal U1. When thyristors 141 and 143 or thyristors 142 and 144 are on, terminals U1 and V1 of the primary of transformer T14 is short-circuited through the conducting thyristors, and no potential difference appears across the terminals U1 and V1. Thus, when thyristors 141-144 are on-off controlled in a suitable sequence and in a prescribed frequency, a variable AC output voltage e14 is outputted from terminals u1 and v1 of the secondary of transformer T14.
Similarly, a variable AC output voltage e16 is outputted from terminals u2 and v2 of the secondary of a transformer T16 in the second unit inverter 16.
A turn-off control or commutation control for GTO thyristors is known. Therefore, illustrations of circuit configurations for the turn-off/commutation control are omitted.
A prior art multiple inverter requires an expensive high-speed breaker 18 which can cut off the output current path of unit inverters within a short period less than about 0.1f (here f denotes the AC output frequency of the multiple inverter). Such a high-speed breaker 18 generally consists of a thyristor electronic switch.
Now, the reason why a high-speed breaker 18 is required will be discussed. Suppose that a shoot-through (commutation-failure) occurs during the actuation of a multiple inverter. Here, the "shoot-through" means that, for instance, thyristor 142 is turned on before thyristor 141 is turned off, resulting in short-circuiting the DC power lines 15P and 15N. When such a shoot-through occurs, even if a reactor (inductor or retardation coil) is connected in series with unit inverter 14, a large discharge current of a thousand amperes could flow from a capacitor C into inverter 14. When the load 20 includes an energy source such as another inverter or a synchronous motor (which may function as an AC generator), an excessive overcurrent may flow from the load 20 side into inverter 14. This is because the impedance of said reactor is very small. Generally, the power-handling capacity of the GTO thyristors used for unit inverter 14 (or 16) is not large enough to escape a secondary breakdown due to such a large discharge current. Therefore, such a large current (overcurrent) is undesirable and should be avoided. The high-speed breaker 18 is therefore necessary to quickly shut-off such an overcurrent flow when a shoot-through occurs.
In practice, it is commonly thought that if no problem due to an overcurrent occurs, a low-speed type breaker is more suitable for the breaker 18 than a high-speed type in view of the manufacturing costs.
Another known approach for avoiding an accidental or faulty overcurrent is a thyristor-turn-off method in which all normal thyristors of the multiple inverter are forcibly turned off when any shoot-through occurs.
However, where the DC input circuits of unit inverters 14 and 16 are connected in parallel as shown in FIG. 2, a large secondary AC current could accidentaly flow when an accident such as a shoot-through occurs. For instance, where thyristors 141, 142 and 144 are simultaneously turned on due to the shoot-through and thyristors 161 to 164 are all turned off by the thyristor-turn-off method, short-circuit paths are formed through the elements V2.fwdarw.D163.fwdarw.141.fwdarw.142.fwdarw.D162.fwdarw.U2, and through the elements V1.fwdarw.144.fwdarw.D142.fwdarw.U1.fwdarw.and V1.fwdarw.D143.fwdarw.141.fwdarw.U1. In this case, the primary winding of each of transformers T14 and T16 is substantially short-circuited. Accordingly, if the load 20 connected to the secondaries of transformers T14 and T16 generates a large counterelectromotive force, an accidental overcurrent temporarily flows from load 20 into unit inverters 14 and 16 until the breaker 18 is cut off. Such a temporary overcurrent is a serious problem when breaker 18 is not the high-speed type.